The purpose of this work was to isolate and characterize the enzyme(s) responsible for processing and eventual degradation of mRNA in avian liver in order to more fully understand the regulatory control mechanism of mRNA levels in eukaroytic cells. An RNase was purified 40,000 fold from chicken liver and its enzymatic properties studied. The enzyme was found to be strongly inhibited by polyadenylic and polyguanylic acids. This inhibition could be reversed by polyamines.